This invention relates to devices which treat tachyarrhythmias (rapid heart rhythms), and more specifically, to methods to provide delivery of atrial cardioversion and defibrillation shocks at appropriate times relative to atrial and ventricular depolarizations.
It has long been recognized that synchronizing atrial and ventricular cardioversion to depolarizations in the chamber being treated improves efficacy of treatment. For example, synchronization of ventricular cardioversion shocks to sensed R-waves is disclosed in U.S. Pat. No. 4,375,817 issued to Engle et al. Synchronization of cardioversion shocks intended to treat atrial or ventricular tachycardia or fibrillation to detected R-waves is disclosed in U.S. Pat. No. 4,384,585, issued to Zipes. Synchronization of atrial cardioversion shocks to detected P-waves is disclosed in U.S. Pat. No. 4,572,191, issued to Mirowski et al.
Delivery of cardioversion or defibrillation shocks intended to terminate a tachyarrhythmia of one chamber unfortunately may induce a tachyarrhythmia in the other chamber. The risk associated with tachyarrhythmia induction in the ventricle is sufficiently great that it has long been recognized that atrial defibrillation pulses need to be timed to avoid the vulnerable period of the ventricle. The most common approach to accomplish this result has been to deliver the atrial defibrillation or cardioversion pulse closely synchronized to a sensed ventricular depolarization to avoid the ventricular vulnerable period, as disclosed in U.S. Pat. No. 4,384,585, issued to Zipes. Due to the fact that in the presence of high ventricular rates, the vulnerable period associated with one R-wave may extend to include the next R-wave, it has also long been recognized that the vulnerable period following a ventricular depolarization may extend to include the time of occurrence of the next subsequent ventricular depolarization in the presence of a sufficiently rapid ventricular rhythm. In such cases, there is no safe time for delivery of cardioversion pulse, as discussed in the article "Synchronous Intracardiac Cardioversion", by Zipes et al., published in Modern Cardiac Pacing, edited by Barold, Futura Publishing Co. 1985, pages 727-743.
Because cardioversion pulses synchronized to a ventricular rhythm which is too rapid may induce ventricular arrhythmias or fibrillation, implantable cardioverters have typically included some method to assure that a minimum R-R interval has elapsed as a prerequisite to delivery of a cardioversion shock. One such synchronization method which prevents delivery of a cardioversion pulse synchronized to a ventricular rhythm which is too rapid is to require that the shock be synchronized to a ventricular depolarization falling outside a defined refractory period defined following the immediately preceding ventricular depolarization, as in the Model 7210 implantable transvenous cardioverter manufactured by Medtronic, Inc. While the device could sense ventricular depolarizations during this refractory period and would initiate a new refractory period following such depolarizations, it would not deliver cardioversion pulses synchronized to such depolarizations,. As reflected in the above-cited article by Zipes et al, the transvenous cardioversion therapy provided by the model 7210 device could be employed to treat either ventricular or supraventricular tachyarrhythmias.
An alternative method for controlling the timing of an atrial defibrillation or cardioversion shock is to deliver the shock after a defined interval following a preceding R-wave, in the absence of a sensed ventricular depolarization, the defined interval being sufficiently long to prevent delivery during the vulnerable period associated with the preceding R-wave. Such a synchronization method is disclosed in U.S. Pat. No. 5,411,524, issued to Mehra. As disclosed in the Mehra patent, the defined interval may vary as a function of the sensed ventricular rate.
In the context of tachyarrhythmia treatment devices capable of sensing in both chambers, the opportunity is presented to require that delivered cardioversion shocks avoid the vulnerable periods of both the atria and ventricles. One device which accomplishes this result is disclosed in U.S. Pat. No. 5,007,422, issued to Pless et al. In the device disclosed in the Pless et al. patent, ventricular cardioversion shocks avoid the vulnerable periods of both chambers by means of the requirement that they are delivered synchronized to a slow ventricular tachycardia R-wave occurring outside the vulnerable period of the atrium. An alternative method to assure that delivered ventricular cardioversion shocks occur outside the vulnerable periods of both the atria and the ventricles is to deliver an atrial pacing pulse and synchronize to an R-wave occurring within 100 ms of the preceding atrial pacing pulse, as disclosed in U.S. Pat. No. 5,074,301, issued to Gilli et al. The same synchronization methods may correspondingly be employed to prevent delivery of an atrial cardioversion pulse during the vulnerable periods of either chamber.